The present invention relates to a new and improved power steering apparatus.
A power steering apparatus is disclosed in U.S. patent application Ser. No. 386,907 filed June 10, 1982 by William T. Rabe and Maurice P. Roberts and entitled "Steering Apparatus", now U.S. Pat. No. 4,418,781. The steering apparatus disclosed in the foregoing application includes a power steering motor having a hollow piston rod which extends from only one side of a piston through a motor cylinder wall. Therefore, the head end working area of the piston is greater than the rod end working area of the piston. The head and rod end working areas of the piston are the surface areas on the head and rod ends of the piston projected into a plane extending perpendicular to a path along which the piston is movable under the influence of fluid pressure. When the fluid pressure to which the head or rod end of a piston is exposed is multiplied by the head or rod end working area of the piston, the result is equal to the force applied to the piston by the fluid pressure.
The steering apparatus disclosed in the aforementioned Rabe and Roberts United States patent compensates for the reduced cross sectional area of the rod end of the piston during operation of the motor by providing a compensator chamber inside the hollow piston rod. The compensator chamber has a working area on the rod end of the piston which is equal to the cross sectional area of the piston rod. A compensator valve assembly is mounted on the piston and directs fluid pressure to the compensator chamber whenever a steering control valve is actuated.
When the steering control valve of the apparatus disclosed in the aforementioned Rabe and Roberts United States patent is actuated to direct fluid pressure against the head end of the piston, the same fluid pressure is also applied against the compensator chamber working area on the rod end of the piston to offset a portion of the fluid pressure applied against the working area on the head end of the piston. As a result, the effective working area of the head end of the piston is equal to the working area on the rod end of the piston. Equal fluid pressure forces are thus applied to the piston during turns in opposite directions when the steering apparatus is subjected to the same operating conditions.
Although the compensator chamber disclosed in the aforementioned Rabe and Roberts United States patent is effective to equalize the rod and head end working areas of the piston during turns in opposite directions, the compensator chamber cannot equalize the rod and head end working areas of the piston when the power steering motor is in an inactive condition and there is a residual fluid pressure in the motor cylinder chambers. At this time, the rod end working area of the piston will be less than the head end working area of the piston by an amount equal to the cross sectional area of the piston rod. Therefore, when the motor is in an inactive condition, the residual fluid pressure in the head end motor cylinder chamber will apply a force against the piston which is greater than the force applied against the piston by the residual fluid pressure in the rod end motor cylinder chamber. This results in a tendency for the power steering motor to be actuated under the influence of the residual fluid pressure even though the steering control valve is in a neutral condition in which the power steering motor should be inactive.